Hybrid electric vehicles or HEV can selectively utilize different energy sources as needed in order to achieve optimal fuel efficiency. An HEV having a full hybrid powertrain can selectively use either or both of an internal combustion engine and a high-voltage battery module or energy storage system (ESS) for electrical propulsion of the HEV. That is, a typical HEV having a full hybrid powertrain can be propelled via purely electrical means, usually upon starting the HEV and speeds up to a threshold speed, with one or more motor/generator units (MGU) alternately drawing power from and delivering power to the ESS as needed. Above the threshold speed, the engine can provide all of the required propulsive torque. By way of contrast, an HEV having a mild hybrid powertrain lacks means for purely electrical propulsion, while retaining certain fuel-saving design features of the full hybrid designs, e.g., regenerative braking capability for recharging the ESS via the MGU and the ability to selectively shut down or power off the engine at idle during Auto Stop events.
The ability of an HEV to automatically shut off or power down the engine, or Auto Stop functionality, allows otherwise wasted fuel to be conserved during certain idle conditions. In a mild HEV having Auto Stop functionality, the high-voltage MGU can be used as a belt alternator starter (BAS) system in lieu of a conventional alternator. The BAS applies torque to a serpentine belt of the engine when a driver signals an intention to resume travel after an Auto Stop event. Torque from the MGU can turn the engine for a transient duration until a flow of fuel from the vehicle fuel supply can be restored. During cold starting of the engine, a conventional crankshaft-mounted auxiliary or 12-volt starter motor can provide the required amount of cranking torque.
Aboard a mild HEV, a high-voltage battery or energy storage system (ESS) supplying high-voltage electrical power to a voltage inverter within the electrical system of the HEV could become temporarily disconnected or otherwise rendered unavailable. This could result in a loss of or an insufficient field excitation, especially for an asynchronous machine-based MGU, which in turn can result in a loss of sustained auxiliary electrical power generation aboard the HEV. Conventional power flow controller and control methods can respond in a less than optimal manner to such a high-voltage electrical fault condition.